To address this problem, Toshiba has developed new SBDs with an improved JBS structure incorporating the concept of the Merged PiN Schottky (MPS) structure. The MPS structure has p+ regions buried in the n- region of an SBD as shown below. (In Toshiba’s design, part of the p-layer of the JBS structure (the shaded area in the figure) is enlarged and the impurity concentration of this part is increased.) The p+ regions and the n- region form a pn junction diode, which turns on when large current (surge current) is needed. This increases the current-carrying capability of the SBD, thereby reducing a rise in forward voltage even at high current and increasing the maximum allowable surge current value.
The MPS structure is characterized by the p+–n-–n+ configuration below the anode electrode.
At low current, the n- region typically has high resistance. However, when this SBD is forward-biased, holes and electrons flow into the n- region from the p and n regions respectively while maintaining electroneutrality. At this time, both holes and electrons exist in the n- region with high concentration. Consequently, the n- region acts like a heavily doped region, particularly at high current, exhibiting very low resistance (conductivity modulation). As a result, this SBD has an IF-VF curve as shown below, with low VF in the high-current region.